Xerographic plate with an inorganic glass binder having an overcoating consisting essentially of aluminum oxide



United States Patent No Drawing. Filed Dec. 21, 1964, Ser. No. 420,170

4 Claims. (Cl. 96-1.5)

ABSTRACT OF THE DISCLOSURE An electrophotographic plate and method comprising a photoconductive layer including an inorganic glass material comprising an inorganic glass binder and finely divided inorganic photoconductive particles dispersed therein, said photoconductive layer having an overcoating comprising a composition selected from the group consisting of germanium dioxide, the oxides of vanadium, titanium dioxide, silicon dioxide, zirconium dioxide, aluminum'oxide; and mixtures thereof. e

This invention relates to electrophotography and more particularly to 7 a novel structure for an electrophotographic plate.

It is known to produce electrostatic images on the sur face of a photoconductive insulating layer by uniformly charging the insulating layer and then dissipating this charge on that portion of the layer which is exposed to light. The latent image formed thereon will correspond to the configuration of the light image passing through the master to be reproduced This image is rendered visible by depositing on the insulating layer a finely divided developing material comprising a colorant called a toner and a toner carrier. The developing material will be attracted to that portion of the layer retaining a charge thereby distributing itself over the layer in a manner corresponding to the electrostatic image. The powder image may then be transferred to paper or other recording surface by placing the surface of the paper in contact with the powdered layer and applying an electrostatic charge to the paper. The paper, upon being separated from the insulating layer, will bear the powdered image which may subsequently be made permanent by heating or other suitable fixing means. This general process is disclosed in detail in United States Patent 2,297,691 to Carlson, and United States Patents 2,357,809 and 3,079,342.

As discussed in Carlson, photoconductive insulating coatings comprise anthracene, sulfur or various mixtures of these materials such as sulfur with selenium, etc., to thereby form uniform amorphous coatings on the base material. These materials have a sensitivity largely limited to the shorter wave lengths and have a further limitation of being only slightly light-sensitive. Consequently, there has been an urgent need for improved photoconductive insulating materials.

The discovery of the photoconductive insulating properties of highly purified vitreous selenium has resulted in this material becoming the standard in commercial xerography. The photographic speed of this material is many times that of the prior art photoconductive insulating materials. However, vitreous selenium suffers from two serious defects: (1) its spectral response is very largely limited to the blue or near ultra-violet; and (2) the preparation of uniform films of vitreous selenium has required highly involved and critical processes, particularly processes involving the preparation of extremely clean and uniform substrates and vacuum evaporation techniques. This, together with the high cost of selenium itself has led, by commercial necessity, to the use of selenium xerographic plates in repetitive processing cycles, that is, it required that the selenium plate be re-used many times in the xerographic process, so that the cost per copy of such a plate may be a reasonably small figure. Under conditions of optimum use, a vitreous selenium plate can be used to prepare 100,000 or even more copies before it deteriorates to the point of unsatisfactory image formation. Under other conditions far fewer copies can be made.

The deterioration observed in selenium plates follows from the mechanical abrasion attendant to the developing process and the cleaning step wherein a rapidly rotating fur brush contacts the selenium surface to remove from the surface any developer particles adhering thereto after the transfer step. In addition to mechanical abrasion, the heat to which the plate is subjected, both by virtue of the friction involved in the various processing steps and, more important, by the propinquity of heat fusing devices generally located (by engineering necessity) in close proximity to the xerographic drum in commercial machines.

In addition, binder plates using resinous binders have been used in elect-rophotography. These resin-binder plates, while desirable as single use plates, have not exhibited the physical hardness for use under long processing cycles. A binder type plate having very desirable reusable properties comprises an inorganic pigment dispersed in a glass binder as disclosed in United States Patent 3,151,- 982. In this patent, the use of a xerographic plate containing a glass binder material of various types of frits is disclosed in detail. In addition, a large number of suitable inorganic pigments were disclosed as being useful together with the above noted glass binder frits. The use of this xerographic plate (as described in US. Patent 3,151,982) has a number of significant advantages over the previously used selenium and other binder type plates. A more desirable spectral response and/ or speed has often been obtained by the use of such glass plates. In addition, a much more convenient commercially adaptable process has been provided than heretofore known in the manufacture of other xerographic plates.

There are, however, some inherent disadvantages to the use of glass binder plates, one being that the glass binder has a tendency to readily absorb atmospheric moisture which adversely affects its reusability and other xerographic properties. When the plate absorbs humidity or moisture, the glass surface exhibits lateral conductivity and this tends to occur at even moderately high relative humidities. This causes a dissipation on the photoconductive surface of the latent electrostatic image formed thereon. This undesirable humidity sensitivity has been especial- 1y noticeable in glass frits which have a relatively high alkali content. In addition, non-alkali-containing glasses have been shown to be humidity sensitive, particularly if they have been subjected to the action of conventional cascading xerographic developers or the action of a polishing abrasive. The Xerographic properties, especially the reusability of these glass binder plates has been seriously hampered by the tendency of these plates to absorb atmospherrc moisture and humidity. Since one of the desirable features of commercial acceptability of glass binder plates resides in their intended reusability, the commercial acceptance of such plates heretofore has been rather limited.

It is therefore an object of this invention to provide a novel glass-containing electrophotog-raphic plate devoid of the above noted disadvantages.

Another object of this invention is to provide a novel overcoating for a glass binder xerographic plate.

'Still another object of this invention is to provide a novel method of making an electrophotographic plate.

Yet another object of this invention is to provide a xerographic plate having a novel photoconductor-overcoating layer combination.

Yet still another object of this invention is to provide a method for increasing and improving the humidity limit of an electrophotographic plate.

Still another object of this invention is to provide a novel method for improving the reusability of the glass binder-inorganic pigment xerographic plate.

Still another further object of this invention is to provide a novel overcoating layer adapted for use with a xerographic plate having a photoconductive layer comprising an inorganic glass.

The foregoing objects and others which will become apparent from the following description are accomplished in accordance with this invention, generally speaking, by providing an electrophotographic plate comprising a photoconductive layer including an inorganic glass material, said photoconductive layer having an overcoating comprising at least one composition selected from the group consisting of germanium dioxide, Geo the oxides of vanadium V V 0 V 0 titanium dioxide, TiO silicon dioxide SiO zirconium dioxide ZrO aluminum oxide, Al 'O and mixtures thereof. The photoconductive layer upon which the overcoating of this invention is deposited may comprise an inorganic glass material which acts as a binder or as the photoconductive material per se. For example, any of the glass-pigment materials defined in US. 3,151,982 may be used as the photoconductive layer in the present invention.

The humidity limit of glass electrophotographic plates has been increased from about 45% to about 65% for alkali glass containing plates and from about 40% to about 65% for abraded non-alkali glass plates. By the term humidity limit is meant, for the purposes of this invention, that amount of humidity that a plate can tolerate and yet be imageable.

The glass containing photoconductive layers and the corresponding electrophotographic plates can be made less humidity sensitive by treatment with vapors of zirconium tetrachloride, vanadium tetrachloride, aluminum trichloride, silicone tetrachloride, germanium tetrachloride and mixtures thereof. At least one surface of the photoconductive layer of a glass containing xerographic plate is exposed in a heated atmosphere to the vapors of these chlorides until the formation of the desired overcoating is completed. It is believed that the reaction of these materials on the surface of the plate involves hydrolysis of the chlorides with atmospheric water, thereby depositing the corresponding oxides on the plate surface and giving off hydrogen chloride. By oxides is meant the oxygen containing composition resulting from the vapor treatment of the plate and is intended to include mono, di, tri and tetra oxides formed by whatever method they are deposited. The thickness of the resulting oxide overcoating layer may vary but generally satisfactory coatings range from about monomolecular to about 2.5 microns depending on the material. However, it Was found that preferred results including good reusability with the retention of high sensitivity of the plates was obtained when an overcoating of from 0.2 to 0.5 micron was used with titanium dioxide. Although any of the listed chlorides function within the present invention, optimum xerographic properties of the plate were obtained when exposing the plate to a composition comprising titanium tetrachloride or aluminum trichloride. The preferred embodiments of the present invention, therefore, comprise a glass binderinorganic pigment xerographic photoconductive layer having an overcoating comprising titanium dioxide or aluminum oxide.

Ordinarily, the overcoating layers of this invention are applied by merely holding the photoconductive glass layer over a source of the particular chloride vapor in a warm air atmosphere. A particularly preferred form of the invention which produces markedly improved results, as illustrated in the following examples, involves applying the chloride vapor in a humidity-free atmosphere such as one which is filled with the chloride vapor. It is believed that this further improvement is achieved because when the chloride vapors are applied in air, they are hydrolyzed to 3,397,982 i g V the corresponding oxide by atmospheric water and de posited on the glass surface whereas when a moisture-free atmosphere is employed, the chlorides are forced to abstract absorbed moisture from the glass surface itself in order to form the oxide. In this preferred technique then, the chloride vapors serve the dual function of removing harmful absorbed moisture from the plate and at the same time, sealing and protecting its surface from moisture abrasion, etc. with the oxidewhich forms simultaneously with moisture abstraction.

The following examples will further define the process, overcoating and plate of the present invention. Parts and percentages are by weight unless otherwise indicated. The examples below specifically illustrate various preferred embodiments of the present invention.

EXAMPLES About 50 parts of titanium chloride (TiCl are added to about 50 parts of CCl.; and agitated until the materials appear to form a homogenous solution. The CCl is used to keep the fuming of the chloride under control. A glass binder plate containing parts of a glass binder of the following composition:

and 10 parts cadmium sulfoselenide pigment is heated to about 350 C. The tray of TiCl -CCl solution is placed near a hood opening and warmed slightly to produce a turbulent stream of white fumes in which is placed the pre-heated glass plate. The plate is kept in motion in the fumes until the first signs of deposition are visible. The deposition is continued until the first color fringes due to light interference are visible. The plate is then cooled, washed with deionized water to remove any possible residual chlorides and rubbed dry with a clean towel. A comparison of the glass plate produced in this example without the overcoating is made to that plate resulting by adding the overcoating of this example. The thickness of the layer is measured and found to be about 0.4 micron.

Five additional glass binder plates having the same composition as the glass binder plate coated with titanium tetrachloride described immediately above are made up and coated with germanium tetrachloride, silicon tetrachloride, aluminum trichloride, vanadium tetrachloride, and zirconium tetrachloride using the same coating procedure with the exception that with aluminum trichloride and zirconium tetrachloride which are solids at temperatures well above room temperature are not blended with carbon tetrachloride prior to the coating operation. Instead, these are merely heated until the solids sublime off to coat the plate. Both of these solids require significantly more intense heating in order to drive off the chloride vapors by sublimation than do the other chlorides. However, they form similar oxide coatings on the plate surface. The plates coated with the oxides of titanium, germanium, silicon, aluminum, vanadium, and zirconium are numbered plate numbers I-VI. Six additional plates made up of a non-alkali glass containing ten parts of cadmium sulfoselenide photoconductive pigment to 100 parts of the following non-alkali glass composition: 18 parts SiO 9 parts B 0 65 parts PhD and 8 parts CdO are also coated with the six diiferent halides according to the aforementioned techniques for comparison with uncoated glass plates of the same composition. The plates are then tested to determine the difference in humidity limit tolerance and abrasion resistance between overcoated and non-overcoated plates. In testing the various plates for humidity tolerance, the plate is mounted in an apparatus very similar to the xerographic oflice copier described in US. Patent 2,945,434 to Eichler. This apparatus is positioned in a controlled humidity chamber which enables the operator to control the relative humidity therein and a range of from about 25% to about 95% relative humidity is made in the test. Copies aremade using the plate at several intervals over the humidity range. By observing and comparing copies made at the various degrees of humidity, an approximate measure of the humidity limit for a particular sample plate is observed. This type of test may be conducted either before and after the oxide overcoating is applied or with only one-half of the plate bearing an oxide overcoating to attain a valid comparison of the results of the overcoating.

With respect to abrasion degradation'from conventional cascading xerographic developer as is used in xerographic oifice copiers such as the one described in the aforementioned Eichler patent, the plate is print tested in controlled humidity with the same type of apparatus used for general humidity limit determination before and after an accelerated abrasion test in which the plate is subjected to the abrasive action of xerographic developer cascading over its surface. For convenience, this test is carried out by masking a portion of the plate with tape to expose only part of its surface to the abrasion test and then the abraded and unabraded portions of the same plate are tested for print quality, with the abrasion time necessary to cause unacceptable copy properties in the plate being a measure of the resistance of the plate to the abrasion which it would be expected to undergo in an ordinary oflice copier.

Example I An unovercoated glass plate made according to the formula above for alkali containing glass plates is print tested and found to operate satisfactorily up to about 40% relative humidity. As the relative humidity is increased above this point, print resolution begins to decrease markedly apparently because of lateral conductivity and unsatisfactory prints are produced.

Example II An unovercoated glass plate containing the non-alkali glass formula described above is print tested at various humidities and found to make acceptable copy up to about 73% relative humidity after which print quality decreases rapidly.

Example III The glass plate of Example II is exposed to the accelerated abrasion test described above, and after three minutes of accelerated abrasion the humidity limit at which the plate will produce acceptable print copy is found to be reduced down to about 55% RH. and after five minutes of abrasion it is found that the plate will not even operate satisfactorily at 40% relative humidity.

Example IV The alkali containing plate of Example I is subjected to the accelerated abrasion testing procedure of Example III and it is found that after five minutes of abrasion, the plate will not even produce satisfactory prints above 30% relative humidity.

Example V A non-alkali glass plate made according to the same formulation as the Example III plate is coated with titanium dioxide by subjecting it to the chloride vapors according to the procedure described above on half of its surface. The humidity limit in coated areas is about 75% relative humidity versus about 73% relative humidity in uncoated areas for the production of acceptable prints. Abrasion of the whole plate in the accelerated abrasion test for three minutes is found to cause a decrease in the humidity limit to about 50% relative humidity in the uncoated areas while the coated areas can still produce high quality prints up to about 65% relative humidity.

Example VI An alkali containing glass plate made according to the formulation used to prepare the plate of Example I is overcoated with titanium dioxide-according to the procedure described above and found to raise the humidity limit from about 40% relative humidity to about 75 relative humidity. This higher humidity limit of the coated plate is found to remain essentially unchanged by two minutes in the accelerated abrasion test.

Example VII A non-alkali glass plate made according to the same formulation used for the Example II plate is treated with aluminum trichloride vapors according to the procedure described above to produce an aluminum oxide coating on one-half of the area of this plate with the other half being masked to prevent coating thereon. Prior to abrasion, the humidity limit to produce acceptable print quality is about 82% relative humidity for both the coated and uncoated portions of the plate. After one minute of abrasion, the uncoated portion of the plate has a humidity limit of about 60% RH. whereas the coated portion is essentially unchanged. Two minutes of abrasion reduces the humidity limit of the coated and uncoated portions to about and 60% relative humidity, respectively.

Example VIH An alkali containing plate made according to the glass plate formula used for Example I is treated with aluminum chloride vapors to produce an aluminum oxide coating thereon. When print tested, it is found to produce good quality prints up to about 85% relative humidity.

Example IX Here again, an alkali containing plate made according to the glass formulation used in the Example I plate is also coated with aluminum trichloride to produce an aluminum oxide coating thereon except that instead of using the procedure described above, the glass plate is placed on an alumina, heat resistant thimble containing the aluminum trichloride. This assembly is placed inside a pair of closed silica trays with only a few small openings to allow for the escape of vapor. Heat is then applied with a hot plate from below the trays under an exhaust hood causing the aluminum chloride to sublime and essentially filling the whole closed tray assembly with aluminum trichloride vapors to the exclusion of air. This plate is print tested and found to produce good quality prints up to relative humidity. After 12 minutes of abrasion in the accelerated abrasion test, the humidity limit at which the plates would still produce acceptable quality prints is only reduced to about 80% relative humidity.

Example X Another plate is made exactly according to the technique of the coated plate of Example IX and found to have produced acceptable quality prints up to a relative humidity in excess of 85% and after 60 minutes of abrasion in the accelerated abrasion testing it is still capable of producing acceptable quality prints at relative humidities in excess of 80%.

It should be understood that any of the above oxide overcoatings may be prepared via materials other than the disclosed chlorides. Also, if suitable, other materials may be used with the disclosed chlorides to form the overcoatings used in the present invention. Other materials and conditions can be substituted in the foregoing examples for the specific materials and conditions set forth therein with similar results.

Although the invention has been desaribed in considerable detail, the foregoing examples are used only for the purpose of illustration. It should be understood that such detail is for the purpose of illustration and not limitation. Those skilled in the art, upon reading the foregoing disclosure, will conceive of other modifications and ramifications of this invention; these are intended to be encompassed within the scope of the present invention.

I claim:

1. An electrophotographic plate having a humidity limit of at least about 65%, said plate consisting essentially of a photoconductive layer including an inoragnic glass binder and finely divided inorganic photoconductive particles dispersed throughout said binder, and an overcoating consisting essentially of aluminum oxide contained on said photoconductive layer,

2. The plate of claim 1 wherein said overcoating is up to about 2.5 microns in thickness.

3. The plate of claim 1 wherein said overcoating is from about 0.2 to 0.5 micron in thickness.

4. A method of imaging an electrophotographic plate having a humidity limit of at least about 65 which comprises providing a plate having a photoconductive layer including an inorganic glass binder having finely divided inorganic photoconductive particles dispersed throughout said binder, said photoconductive layer having an overcoating consisting essentially of aluminum oxide, imaging said plate by forming a latent electrostatic image on the surface of said plate, and developing said electrostatic image whereby a visible image is formed on the surface of said plate.

References Cited UNITED STATES PATENTS 1,964,322 6/1934 Hyde 6530 X 2,886,434 5/1959 Owens 961.5 3,288,604 11/1966 Corrsin 96--1 2,860,048 11/1958 Deubner 96-15 I. TRAVIS BROWN, Primary Examiner. C. E. VAN HORN, Assistant Examiner. 

